Apparatus for making plastic film

ABSTRACT

Plastic film is made using a heat transfer liquid circulated through the film extrusion die to control (by either heating or cooling, as required) the temperature of the plastic melt flowing through such die. A plastic bubble emerging at the outlet of the die is inflated and ventilated by continuously circulated gas which is controlled in temperature as it enters and leaves the bubble so that it does not adversely affect the temperature of the melt in the die.

United States Patent [191 Hinrichs APPARATUS FOR MAKING PLASTIC FILMDonald R. Hinrichs, Waynesboro, Va.

Assignee: Reynolds Metals Company, Richmond, Va.

Filed: Apr. 2, 1973 Appl. No.: 347,066

Related U.S. Application Data Division of Ser. No. 126,831, March 14,1971, Pat. No. 3,752,635.

Inventor:

U.S. Cl 425/326 R, 425/379, 425/381 Int. Cl B29f 3/04 Field of Search425/325, 326, 379, 380,

References Cited UNITED STATES PATENTS 11/1966 Mercer 425/380 [451 July9, 1974 3,427,375 2/1969 Turner 425/326 B X 3,491,406 1/1970 Davidson425/326 X 3,570,062 3/1971 Dukert et a1 .1 425/326 X FOREIGN PATENTS ORAPPLICATIONS 439,602 7/1967 Switzerland 164/84 Primary ExaminerR.Spencer Annear [5 7] ABSTRACT Plastic film is made using a heat transferliquid circulated through the film extrusion die to control (by eitherheating or cooling, as required) the temperature of the plastic meltflowing through such die. A plastic bubble emerging at the outlet of thedie is inflated and ventilated by continuously circulated gas which iscontrolled in temperature as it enters and leaves the hub ble so that itdoes not adversely affect the temperature of the melt in the die.

11 Claims, 9 Drawing Figures 1 APPARATUS FOR MAKING PLASTIC FILMBACKGROUND OF THE INVENTION Plastic film has long been made by inflatinga tubular section being extruded from an annular orifice in a die todefine a tubular film bubble and then collapsing the tube between a pairof cooperating rollers to define a double thickness film which is rolledon a take-up roll. Control of the temperature of a plastic melt flowingthrough an extrusion die has been a problem. Degradation of the meltwithin the die or build-up of solidified melt along the inner lip of thedie outlet can have adverse effects on film quality and shorten theoperating cycle of the die before the die must be shut down forrecleaning of the die passages. Temperature control is important forachieving uniform gauge across the width of the film strip afterslitting of the extruded tube. Any lack of uniform gauge is magnifiedwhen the film is subsequently stretched, as it frequently is forpurposes of makingoriented heat-shrinkable film.

SUMMARY This invention provides an improved apparatus and method forextruding plastic film in a more economical manner, due to longeroperating cycles between shutdowns, and with improved uniformity ofgauge transversely of the direction of extrusion. This apparatus andmethod employs a temperature and flow controlled heat transfer liquidwhich is circulated through the extrusion die to control (by eitherheating or cooling, as required) the temperature of the plastic meltflowing through the die. Air or other gas used to inflate and ventilatethe plastic bubble emerging from the die outlet is also controlled intemperature when it passes through the die so as to not adversely affectthe temperature of the melt in the die.

Other details, uses, and advantages of this invention will becomeapparent as the following description of the embodiment thereofpresented in the accompanying drawings proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings show apresent preferred embodiment of this invention, in which:

FIG. 1 is a schematic side elevation, partly sectioned and partly brokenaway, of a film extrusion die and associated temperature controlapparatus, including the emerging tubular film bubble and pinch rolls;

FIG. 2 is a perspective view schematically illustrating the overall flowof fluids through the extrusion die of FIG. 1;

FIG. 3 is a schematic and partially broken away fragmentarycross-section view taken on the axis of the film extrusion die shown inFIG. 1;

FIG. 4 is a cross-sectional view taken essentially on the line 4-4 ofFIG. 1;

FIG. 5 is a schematic and partially broken away fragmentarycross-section view taken approximately on the line 5-5 of FIG. 4 to showthe flow passages of the liquid and gas between the channels carryingthe plastic melt;

FIG. 6 is an enlarged fragmentary perspective view particularlyillustrating the detailed construction of a stranding chamber andcombining chamber comprising the exemplary extrusion die of thisinvention;

FIG. 7 is a fragmentary cross-sectional view taken essentially on theline 7-7 OF FIG. 4;

FIG. 8 is a greatly enlarged fragmentary view particularly illustratingthe cross-sectional configuration of a part of the stranding chamber;and

FIG. 9 is a view similar to FIG. 8 of the combining chamber.

ignated generally by the reference numeral 10 and is adapted forextruding polyvinyl chloride, nylon, or

other polymeric or like plastic material to produce a thin plastic film.

The apparatus 10 comprises an extrusion die 11 having an inlet 12 and anannular outlet 13 and an extruder 14 supplies a hot plastic melt 15under pressure to the inlet 12. A supply bin 16 feeds the extruder 14with a suitable plastic material 17 in flowable particulate form; and,the extruder 14 has heating means for melting the plastic material 17and a helical screw 20 for forcing the molten material or melt 15through an outlet 21 and through a supply conduit 22 to the die inlet12. The plastic melt 15 is supplied to the inlet 12 at a controlled andsubstantially constant temperature suitable for the physical andchemical properties of the plastic material 17.

The apparatus 10 has means indicated generally at 23 for supplying airor other suitable gas (designated by dotted arrows 24) through the die11 to inflate a tube produced by extrusion of the plastic melt 15through the annular die outlet 13, and such tube is pinched by rollers26 to trap a moving film bubble 25 between the die outlet and therollers. The double thickness of film passing from the rollers 26 may bewound on a supply roll or further processed in any suitable manner knownin the art. The tube is slit and the film is stretched to increase itsarea and then chilled to retain it in stretched condition. It is then incondition for subsequent heat shrinking for packaging purposes.

The air 24 is suitably supplied to the die 11 and is conveyed axiallytherethrough using circumferentially spaced passages 27, see FIG. 3. Theair is then conveyed by radially inwardly directed passages to a centralcylindrical passage 30 which extends substantially axially through theupper end portion of the die 11 into the film bubble 25. The air 24 issupplied at a controlled temperature and pressure to cause expansion ofthe film bubble 25 in a precisely controlled manner.

The air 24 flows into the center of bubble 25 through passage 30 andinitially flows upwardly into such bubble as indicated by the arrows at31. It then flows downwardly along the sides of the bubble (as indicatedby the arrows 32) into a plurality of passages 33 through the die whichextend in spaced parallel relation to each other and are arrangedconcentrically around the central passage 30. The passages 33communicate with an annular chamber 34 and one or more passages 36 areprovided in die 11 in flow communication with chamber 34 for conveyingthe air in such chamber out of the die.

Volatile substances from the plastic melt pass into the air in thebubble, and tend to build up in concentration as more film passes aroundthe bubble. Moreover, cold air is blown (by conventional means, notshown) against the outside of the emerging extruded film to chill andsolidify it. These effects can lead to condensation of the volatilesubstances and consequent damage to the film upon being contacted by thecondensate. Continuous withdrawal of air in the bubble 25 prevents suchbuild up of concentration of volatiles and, thus, protects the film fromthe damaging effects of excessive concentration of volatiles.

The apparatus and method of this invention uses a heat transfer fluid inthe form of a liquid designated by solid arrows 35. The liquid 35provides efficient temperature control of the melt in the die by eitherheating or cooling the melt, as required in different parts of the die.In addition, the liquid 35 is also used (as will be describedsubsequently), to control the temperature of the air 24 entering andleaving the bubble 25 to further assure precise control of thetemperature and, hence, even flow of the melt through the die 11. Theliquid 35 is supplied to the die from a manifold 12 which encircles thedie.

While the liquid 35 might for heating purposes be replaced by fluids inother phases, such as gases or gaseous vapors (e.g., steam), a liquid isfar superior because it can function efficiently both for heating andcooling, which is part of the intended operation of the apparatus andmethod of the invention. Steam and like vapors may be suitable forheating but are unsuitable for cooling in dies of the kind in question.Gases like air are inefficient for both heating and cooling in dies ofthe kind in question. because of poor heat absorption and heat transfercapacity.

During operation of the apparatus 10, the melt 15 enters inlet 12 andtravels vertically upwardly through a cylindrical passage 44 in a lowerportion 11A of die 11. The melt l5 flares outwardly from the top ofpassage 44 into an annular conically extending passage 45 in the dielower portion 11A. The die 11 has a stranding chamber 46 in flowcommunication with the peripheral edge of the passage 45; and aplurality of spaced parallel channels 47 extend through a centralportion 11C of die ll and at their lower ends communicate with theperipheral edge of the stranding chamber 46 and convey the melttherefrom in a plurality of separated plastic strands 48 through thecentral die portion 11C. The strands 48 are recombined in a combiningchamber 49 in the upper portion 118 of the die where the upper ends ofchannels 47 communicate with chamber 49. The chamber 49 communicateswith the lower end of an annular cylindrical passage 50 through theupper die portion 118. The upper end of passage 50 terminates in theannular die outlet orifice 13.

Die 11 has integral heat transfer means comprising a plurality ofcooperating passages in the die body. The lower portion 1 1A of the die11 has passages 51 to provide circulation of liquid 35 closely adjacentthe melt passing into and through conical passage 45. A plurality ofinterconnected passages 52 provide flow of liquid 35 through parts ofthe central die portion 11C and circumferentially around portion 11Cwhere it extends around the outside of the strand channels 47. Otherinterconnected passages 53 provide circulation of liquid 35 through theupper die portion 118 where it extends around the outside of the annularpassage 50. In addition, interconnected passages 54 provide flow ofliquid 35 through the central and upper die portions 11C and 11B andhave portions which pass closely adjacent to the strand channels 47 andannular passage 50. The passages 54 also extend closely adjacent toreturn airconveying passages 33. The interconnected passages 54 of thisexample are placed in flow communication with the passages 52 throughthe use of radial passages 55. The passages 55 extend radially throughthe central die portion 1 1C and provide flow of liquid 35 between pairsof strand channels 47 and both to and from passages 54. The liquid 35 iscollected from the die in manifold 124 which encircles the die.

The liquid-conveying passages 51-55 have been shown schematically inFIG. 2; and, it will be appreciated that this has been done forsimplicity of presentation. Further, in some applications it may bedesired to provide liquid to the passages 54 independently of thepassages 52.

FIG. 5 shows a schematic arrangement of air and liquid passages in thedie as air and liquid pass continuously between the strand channels 47.In the present preferred embodiment of the invention, a series of strandchannels designated 47A, 47B, 47C and 47A have different flow passagesfor air and liquid between them, and the flow passage pattern isrepeated for each succeeding set of four channels (channel 47A being thefirst channel of the next series of four) and so on around the die.Between channels 47A and 478 there is a radially extending air outflowpassage (indicated by flow arrows designated 36A) connecting passages 33and 36. About midway between channels 47B and 47C there are radiallyextending liquid outflow and inflow passages (indicated by flow arrows55 The liquid outflow and inflow passages are displaced vertically toavoid interference with each other. Incoming preheated air 27A is fedbetween channels 478 and 47C (shown close to channel 47C for conveniencein the figure, but actually midway between channels 478 and 47C anddisplaced vertically to avoid interference with the liquid passages 55).The arrangement of air and liquid passages between channels 478 and 47Cis repeated between channels 47C and 47A. The pattern is repeated forthe next series of four channels as explained above, wherein 47A is thefirst channel of the next series.

The liquid 35 thus passing back and forth between strand channels 47controls the temperature of the metal between strand channels 47 tothereby control the temperature and flow of the melt 15. The liquid 35also flowing through those portions of passages 54 arranged adjacent airpassages 33 supplements the reheating of cooled returning bubble air inpassages 33 for purposes of suppressing undue local cooling effect onthe melt of the returning air as it passes between strand channels 47.

The incoming air 24 provided to the die 11 is preheated to a temperatureclose to the temperature of the melt and the liquid 35 flowing throughthe various interconnected liquid-conveying passages 51-55 assures thatthe preheated air temperature is equalized with that of the melt as theyboth flow through die 11. In addition, these liquidconveying passagesare supplied with liquid 35 at the same predetermined temperature,preferably by being connected to a common preheated reservoir and this.tends to equalize temperatures throughout the die.

In extruding melt through an associated die the temperature of the meltadjacent the die outlet is greater than the temperature of such melt atthe die inlet. To

prevent the melt outlet temperature from becoming excessive, which wouldcause rapid degradation and possible charring of the melt resulting inblockage of the melt passages, the temperature of the liquid 35 flowingthrough passages 53 and54 in the upper die portion 11B, for example, isrelatively cooler than the temperature of the melt as it flows throughdie portion 11B whereby such liquid serves to locally cool the melt.

The exemplary apparatus has its supply of heat transfer liquid 35contained in a reservoir 56, see'FIG. 1, and a system is provided forsupplying the liquid 35 to the die 11 and returning such liquid to thereservoir 56 and such system comprises a supply conduit system 57, areturn conduit system 58, and suitable pressure means in the form of apump 60 driven by a motor 61 for flowing or circulating the liquid 35through the conduit systems 57 and 58 and, hence, die 11. The apparatus10 also has a heat transfer assembly 62 which is provided with heatexchangers and suitable controls for,

controlling the temperature and flow of liquid 35.

The air 24 supplied to the die 11 for inflating the hubble 25 ispreheated in a heat exchanger 64 which is supplied with liquid 35 at acontrolled temperature from the assembly 62. The heat'exchanger 64 is inthe form of an oil-to-air heat exchanger and is supplied with air from acompressor 65. However, the air 24 may be preheated using any suitabletechnique or may be partially heated prior to being introduced into theheat exchanger 64. An adjustable air pressure regulator 66 is alsoprovided in a supply conduit 67 which provides the air 24 to the inletpassages 27 of the die and the regulator 66 may be adjusted to assureproper inflation of the bubble 25.

In this example, liquid at a predetermined temperature and fromreservoir 56 is suppliedto all parts of the die 11. The die 1] maybesupplied with suitable external conduits so that the flow of liquidthrough its portions 11A, 11B, and 11C may be in series, parallel, or inseries through certain die portions and in parallel through the otherportion and vice versa. In addition, the use of parallel flow pathsthrough the die makes it possible to supply liquid at differenttemperatures and, if desired, from separate reservoirs'through each flowpath whereby in one portion of the die the liquid 35 provides heatingwhile in another portion of the die such liquid provides cooling.However, regardless of whether a liquid 35 is supplied to each dieportion from a single reservoir or a plurality of reservoirs, by havingparallel flow paths through the die the rate of liquid flow through eachparallel path may be controlled by the configurations and sizes of theflow passages and the use of suitable valves and restrictors whereby thedesired amount of heating or cooling provided by the liquid 35 in theassociated die portion may be precisely controlled. v

To enable easy comprehension of the overall operations of the die 11 theabove description has been made referring to only FIGS. 1 and 3 of thedrawings. The presentation will now proceed with a more detaileddescription of the construction and operation of the component partscomprising the exemplary die ll.

As best seen in FIG. 4, the die 11 has a fixed lower or bottom portioncomprised of a die stator 70 and a slip ring assembly 73, to besubsequently described in more detail, which allow rotation of theentire remaining upper portion of die 11 relative to the nonrotatingfilm bubble 25 and this compensates for any irregularities or defectswhich might be present in the annular orifice 13, for example, therebyassuring that rolls wound from the film will have uniform thicknessthroughout. Uneven flow of the plastic melt through the die 11 issubstantially eliminated due to the unique design of its melt flowpassages and the precise control of the temperature of the flowing melt.

The die stator has a catch basin 71 suitably fixed thereto by bolts 72.The slip ring assembly 73 is comprised of a central slip ring 74, drivebars 75, and an upper segmented ring 76, with the members 75 and 76being fixed to the central ring 74 by bolts 77 and 78 respectively.

The stator 70 and assembly 73 are restrained from rotating by a suitableframe or supporting structure and the heat transfer liquid 35 for thedie 11 is provided through an inlet 79 in the stator 70 whichcommunicates with an annular chamber 79A. The air 24 used to expand thebubble 25 is provided to the die 11 thorugh an inlet connection 27A inthe assembly 73 which supplies air to an annular chamber 27B and thepreviously described passages 27 are in flow communication with chamber27B. With this arrangement it is possible to provide continuous rotationof the upper part of the die 11 without concern for the air and heattransfer liquid connections.

The die 11 has an insert and hub portion designated generally by thereference numeral 80 which is comprised of a central member 81 which haspassage 44 extending axially therethrough in the form of a rightcircular cylindrical opening. A member 82 is fastened concentricallyaround member 81 to define an annular cylindrical passage 83 used toconvey liquid 35 closely adjacent the column of melt 15 in passage 44.The lower portion of the member 82 is suitably threadedly fastened to aring 84.

A die rotor 85 is threadedly fastened to the member 82 as indicated at86 and the die rotor 85 with the insert and hub assembly 80 fixedthereto is provided with a rotary bearing assembly 87. The upper part ofdie 11 may be easily rotated (while keeping the die stator 70, catchbasin 71, and assembly 73 stationary, as previously mentioned) merely byrotating member 82 through a threaded connection 90 provided in thelower portion of the die 11. Although means have not been shown forrotating the upper portion of the die 11, it will be appreciated thatany suitable means known in the art may be used for this purpose.

Suitable fluid seals are also provided, as shown in FIG. 4, to assurethat neither air 24 nor heat transfer liquid 35 leaks out of the die 11between the die stator 70, assembly 73, and adjacent rotatingcomponents.

The die 11 has what will be referred to as a die base 91 suitably fixedto the die rotor 85 and, hence, assembly 80 by a plurality of bolts 92and a roughly frustoconical female surface 93 defines the top surface ofthe die base 91. The surface 93 cooperates with a substantiallyfrustoconical male surface 94 comprising a member which will be referredto as a mandrel shell 95 to define the previously mentioned passage 45.The mandrel shell 95 comprises a part of both the upper die portion 11Band the central die portion 11C.

The die 11 has a member 96 supported in the central portion of themandrel shell 95 and member 96 comrpises the upper portion 11B of suchdie and has the passages or bores 33 defined therein. The member 96 alsohas theright circular cylindrical air passage 30 defined substantiallycentrally therethrough and the passages 33 are arranged in spacedrelation concentrically around the central passage 30, see FIGS. 3-5.The member 96 is held in position by a mandrel cap 100 which is attachedto the mandrel shell 95 by a plurality of threaded bolts 101. Suitablefluid sealing rings are provided between the members 95, 96 and 100 toassure that the heat transfer liquid 35 and air 24 do not leak into eachothers passages.

The die 11 has a pad ring 102 supported concentrically around the lowerportion of the mandrel shell 95 so as to define the annular cylindricalvolume or passage 50 therebetween; and, the die 11 also has a cup ringassembly 104 comprised of a top cup ring 105A and a bottom cup ring 105Bfixed together by a plurality of bolts 106. The assembly 104 is fixed tothe die rotor 85 by a plurality of bolts 107 which have plain centralportions of extended lengths and threaded end portions. Each centralportion of each bolt extends through aligned bores 108, 109, and 110 inthe die rotor 85, die base 91, and mandrel shell 95 respectively whichhave larger diameters than the outside diameter of an associated bolt107 whereby an annular air discharge passage 36 of extended length isdefinedbetween each bolt 107 and its associated aligned bores.

The die 11 has radially extending air discharge passages 36A in member95, which are shown by dotted lines in FIG. 4 and schematically in FIG.5, and such passages communicate with chamber 34 and the upper ends ofpassages 36. The lower ends of air passages 36 communicate with anannular chamber 36B through radially extending passages and an outletconnection 36C is provided for discharging the air 24 out of the die.

The threaded end portions of bolts 107 are threadedly fastened in thebottom cup ring 1058. Thus. it is seen that the top outer portion 11B ofthe die is held to the die rotor 85 by bolts 107 and unique airdischarge passages 36 are provided around the outside surfaces of suchbolts.

The top cup ring 105A has an inside cylindrical surface 112 whichcooperates with an outside cylindrical surface 113 of the mandrel shell95 to define the upper portion of annular passage 50 and such upperportion terminates in the annular die orifice 13.

A header ring 115 is provided at the top of the die 11 and is fixed inposition at the top of and concentrically around the shell 95 bywelding. The previously mentioned interconnected passages 53 areprovided in the assembly 104 comprising the upper die portion 11B andcirculate heat transfer liquid 35 adjacent the outside surface of themelt as it is extruded in tubular form through the passage 50.

The die 11 has a manifold assembly or manifold 120 comprised of a ring121 having a roughly T-shaped cross-sectional configuration and an outercylindrical ring 122 which is fixed to ring 121 in a fluid-tight manneras by welding, or the like. The manifold 120 is held in position betweenthe die base 91 and the cup ring assembly 104 and suitable fluid sealsare provided between the mandrel shell 95 and manifold 120 to preventescape of liquid 35 from the manifold 120.

The manifold 120 has a pair of substantially toroidal chambers 123 and124 provided therein which provide flow of heat transfer liquid 35circumferentially around the central die portion 11C. The chambers 123and I24 communicate with passages 123A and 124A respectively and thechambers 123-124 and passages 123A-124A comprise the previouslydescribed interconnected passages 52.

The previously mentioned radially extending liquid conveying passages 55communicate with passages 123A and 1248 and assure that liquid 35 isprovided adjacent each strand channel 47. In particular, reference ismade to FIG. 5 of the drawings where it is seen that a plurality of fourstrand channels 47 have been shown defining three zones X, Y, and Zbetween immediately adjacent pairs of channels. Heat transfer liquid 35flows both inwardly and outwardly between channels 47 defining zone Xwith air 24 being supplied from passage 27 through radial passages 27C,also see FIG. 4, and into passage 30. The flow of liquid 35 and air 24between the channels 47 defining zone Y is similar to the flow in zoneX. Only air flow is provided between the channels 47 defining zone Z andsuch air 24 is from the bubble 25 and has passed through passages 33 andchamber 34 whereupon it is conveyed by radial air passages 34A to airdischarge passages 36. Following zone Z, zones X, Y and Z are repeatedaround the entire circumference of the die.

As will be apparent particularly from FIGS. 2 and 4 of the drawings, thepassage 45 is provided with a plurality of radially inwardly directedpassages 125 adjacent thereto which are angularly spaced only a fewdegrees apart, essentially as shown in FIG. 2. The passages 125 aresupplied with liquid 35 from an annular chamber 126 which is in turnsupplied with liquid from inlet 79. The passages 125 communicate with achamber 127 which supplies liquid to passage 83. The passages 83 and 125together with chambers 126 and 127 comprise the plurality ofinterconnected passages 51 previously described in connection with FIG.3 and such interconnected passages assure precise temperature control ofthe melt 15 in passages 44 and 45.

As previously mentioned, the die 11 has a stranding chamber 46 and acombining chamber 49 and such chambers are arranged at opposite ends ofthe strand channels 47, see FIGS. 6-9. The chamber 46 is defined bycooperating surfaces in the die base 91 and the mandrel shell 95. Thechamber 49 is defined by cooperating surfaces in the mandrel shell andthe lower inside portion of the pad ring 102. The chambers 46 and 49 arevery similar and fragmentary portions of these chambers are shown inFIGS. 8 and 9 respectively; therefore, similar component parts thereofwill be given identical reference numerals and subsequently describedonly in connection with the stranding chamber 46 with it beingunderstood that a similar description is fully applicable to thecombining chamber 49.

The chambers 46 and 49 help assure smooth flow of viscoelastic materialthrough the die at a substantially constant shear rate, even though themelt 15 is being stranded or separated into a plurality of spacedstrands in the chamber 46 and combined in a tubular form in chamber 49.The flow of melt 15 at a substantially constant shear rate is also madepossible through the use of the integral heat transfer passages providedin the die 11 which assure that the melt flowing through such die iskept at a substantially constant temperature by the action of the liquid35 providing either heating or cooling, as required, and by heating theair 24 which flows out of the bubble 25 to prevent cooling of theplastic melt by'such air as it exits the die 11.

Each chamber 46 and 49 has a pair of opposed walls or surfaces 133 and134 and those portions of walls 133-134 which are arranged nearer thecenter of the die 11 are arranged approximately horizontally and insubstantially parallel relation. The chamber 46 also has an undulatingperipheral surface which is designated generally be thereference numeral131 in FIG. 6 and extends approximately vertically between walls 133 and134. The surface 131 is defined by a plurality of spikes orsubstantially V-shaped surfaces 135 interconnected at their bases byarcuate surfaces 136 which may be substantially semicylindrical in someapplicatrons.

The V-shaped surfaces 135 are provided with a fillet 137 extending in asmooth are between surface 133 and surface 134'. A fillet 137 is alsoprovided between arcuate surface 136 and surface 133 and in this latterinstance, such fillet has a substantially elliptical crosssectionalconfiguration. The fillet 137 further assures smooth flow of meltthrough its associated chamber. As will be apparent from FIG. 6, each ofthe surfaces 136 is aligned with an associated roughly semicylindricalwall portion of a strand channel 47 whereby the projections 135, arcuatesurface 136, and fillet surfaces 137 provide smooth converging flow ofmelt from stranding chamber 46 into thestrand channels 47. Similarsurfaces in the combining'chamber 49 provide smooth diverging flow ofthe melt into the annular passage 50 which communicates with thedischarge orifice 13.

The spaced bores 47 preferably have circular crosssectionalconfigurations throughout their lengths-and, in this example, each ofthese bores has a tapered or frustoconical configuration. Thefrustoconical configuration may be precisely finished using taperedhelical reamers to provide a smooth surface which assures the plasticmelt will flow through the die with minimum resistance and stagnation.

The heat transfer liquid 35 is of the type which has a relatively highthermalconductivity and a high boiling point. One liquid which has beensuccessfully used is sold under the trade nume of THERMlNOL FR-l and ismanufactured by the Monsanto Company, St. Louis, Missouri.

The improved apparatus and method of this invention has been presentedin connection with the making of polyvinyl chloride film using anassociated melt;

however, it is to be understood that this invention may be used to makefilm using any suitable viscoelastic material capable of being extrudedthrough an extrusion die.

It has also been found that temperatures of the die 11 at mostmelt-wetted surfaces are within one or two degrees Fahrenheit of thetemperature of the heat trans fer liquid flowing through the die, asdetermined by actual measurements. Further, the melt-wetted surfaceshave been found to vary no more than roughly plus or minus lF. over therange of heat loads normally encountered in processing.

The apparatus and method of this invention assures precisely controlledflow of a viscoelastic material through the die 11 under optimumconditions and such controlled flow is provided because the shear rateof the melt at the wall of the channel is maintained substantiallyconstant due to the unique design of the melt flow channels and theprecise control of the temperature of the melt. Thus, the plastic filmextruded from the die 11 is of optimum quality and hence substantially10 free of thickness buildups, striations, imperfections,discolorations, or holes.

in atypical application using the apparatus and method of this inventionto make polyvinyl chloride film, the die 11 had an outlet orificediameter of 10 inches and the die 11 had anoutlet orifice diameter of 10inches and the die had 72 strand channels. Melt was provided fromextruder 14 so that it entered the inlet 12 of die 11 at a flow rate ofapproximately 400 lbs. per hour, at a temperature of approximately420F., and a pressure of approximately 3,500 psi. The melt extrudingfrom the outlet orifice 13 was at a temperature of approximately 435F.

It has been found by tests that the melt temperature rises in the diedue to dissipation of the pressure at the rate of about 6.5F. per 1,000psi. Although some of the heat is lost to the cooler die surfaces, itwill be readily apparent that there is generally a need to provide forlocal cooling of the melt particularly in local hot spots near the dieoutlet.

The heat transfer liquid 35 in this application entered the die at atemperature of approximately 420F. and a pressure of approximately 20psi and exited the die at approximately 420F. and 15 psi.

The air 24 to the die was controlled so that it was heated by the heatexchanger 64 and entered the die 11 at a temperature of approximately390F. and a flow rate of about 10 cfm. The air 24 exited the die atessentially the same temperature and flow rate. The air entered thebubble 25 at a temperature of approximately 380F. and 10 cfm and exitedthe bubble at an estimated temperature of 200F. and 10 cfm whereupon itwas reheated in the die so that it exited the die at the previouslymentioned temperature of about 390F. and flow rate of 10 cfm so as notto adversely affect the temperature of the melt.

It has been found that polyvinyl chloride film made using the apparatusand method of this invention has film thickness variations which areregularly and significantly less than one-half the thickness variationsfound in polyvinyl chloride film made using previously proposedapparatus and methods.

While present exemplary embodiments of this invention, and methods ofpracticing the same, have been illustrated and described, it will berecognized that this invention may be otherwise variously embodied andpracticed within the scope of the following claims.

What is claimed is:

1. An apparatus for making plastic film comprising, an extrusion diehaving an inlet, a plurality of spaced channels in said diecommunicating with said inlet, an annular outlet communicating with saidchannels, said die being adapted to receive a plastic melt in said inletwhich is separated into spaced strands in said channels and combinedprior to exiting ,said outlet, means providing said plastic melt to saidinlet under pressure causing the extrusion of a plastic tube closeableat a position spaced from said outlet, means for supplying a gas throughsaid die to inflate said tube to form a plastic bubble which is used todefine said film, heat transfer means in said die comprising a pluralityof spaced passages extending adjacent and parallel to said spacedchannels, and a system for providing a heat transfer fluid at acontrolled temperature and flow through said heat transfer means tocontrol the temperature of said melt by either heating or coolingthereof and thereby more precisely control the flow of said melt.

2. An apparatus as set forth in claim 1 in which said spaced channelscomprise a plurality of parallel channels extending axially through saiddie.

3. An apparatus as set forth in claim 2 in which said die has an inletportion, an outlet portion, and a central portion arranged therebetweenand said plurality of spaced passages extend through said centralportion.

4. An apparatus as set forth in claim 1 in which said system comprises asystem for providing a liquid which is capable of either heating orcooling said melt.

5. An apparatus for making plastic film comprising, an extrusion diehaving an inlet and an annular outlet, means providing a plastic melt tosaid inlet under pressure causing the extrusion of a plastic tubecloseable at a position spaced from said outlet, means for supplying agas through said die to inflate said tube to form a thinwalled plasticbubble which is used to define said film, heat transfer means providedas an integral part of said die for controlling the temperature of saidgas passing through said die, and a system for providing a heat transferfluid at a controlled temperature and flow through said heat transfermeans to control the temperature of said melt by either heating orcooling thereof and thereby more precisely control the flow of saidmelt.

6. An apparatus as set forth in claim 5 in which said system comprises asystem for providing a liquid which is also capable of either heating orcooling said gas.

7. In combination: a die for extruding plastic film having an inlet, anannular outlet, and integral heat transfer means comprising a pluralityof liquidcoveying passages in said die; means providing a plastic meltunder pressure to said inlet causing the extrusion of a plastic tubefrom said outlet; said tube being closeable at a position spaced fromsaid outlet; means for supplying a gas through said die to inflate saidtube to form a plastic bubble which is used to define said film; aplurality of gasconveying passages in said die for receiving gas exitingsaid bubble, said gas-conveying passages being arranged adjacent andparallel to portions of said liquid-conveying passages; a heat transferliquid; a system for providing said liquid through said liquidconveyingpassages and heat transfer means; and means for controlling thetemperature and flow of said liquid to control the temperature and flowof said melt while simultaneously heating said exiting gas to atemperature which prevents cooling of said melt upon flowing saidexiting gas adjacent said melt and out of said die.

8. A die for extruding plastic film, comprising an inlet opening forreceiving a plastic melt under pressure, a passage communicating withsaid inlet opening, a plurality of spaced channels having inlet endscommunicating with said passage, and an annulus having an inlet endcommunicating with the outlet ends of said channels and a dischargeorifice for extruding the melt in tubular form, a first series ofpassages through the die extending inwardly between said channels andthence out of a portion of the die surrounded by said annulus, saidfirst series of passages being adapted to supply heated gas continuouslyto inflate the tubular film emerging from the orifice, and a secondseries of passages ex tending through a portion of the die surrounded bysaid annulus and thence outwardly between said channels, said secondseries of passages being adapted to withdraw gas continuously fromwithin the inflated tubular film and thereby control the gaseous contentof the inflated film.

9. The die of claim 8, in which each gas passage is substantiallyequidistant between the pair of adjacent channels through which itpasses.

10. The die of claim 9, in which the gas inlet passages and gas outletpassages pass between different pairs of channels.

11. An apparatus for making plastic film, comprising; an extrusion diehaving an inlet; a passage in said die communicating with said inlet; astranding chamber in said die adjoining the peripheral edge of saidpassage; a plurality of spaced substantially axially extending channelsin said die communicating with said stranding chamber and each having adischarge end; a combining chamber in said die communicating with thedischarge end of each spaced channel; each of said chambers having anundulating peripheral surface defined by a plurality of substantiallyV-shaped surfaces interconnected at their bases by arcuate surfaces witheach arcuate surface being aligned with an associated end portion of anassociated channel; an annular discharge orifice in said diecommunicating with said combining chamber; means providing a plasticmelt to said inlet under pressure causing the extrusion of a plastictube from said orifice which is closeable at a position spaced from saidoutlet; said plastic melt being received in said inlet as a solid columnwhich is spread radially in said passage, separated into spaced strandsin said channels, combined in said combining chamber to effectivelyminimize uneven flow rates in different parts of said die, and extrudedfrom said orifice to define said plastic tube; said V-shaped surfaces insaid stranding chamber providing smooth converging flow of plastic meltinto said channels and said V-shaped surfaces in said combining chamberproviding smooth diverging flow of said melt into an annular passagecommunicating with said annular discharge orifice; means for supplying agas through said die to inflate said tube to form a plastic bubble whichis used to define said film; heat transfer means in said die comprisinga plurality of spaced passages extending adjacent and parallel to saidspaced cahnnels; and a system for providing a heat transfer liquid at acontrolled temperature and flow through said heat transfer means tocontrol the temperature and flow of said melt.

1. An apparatus for making plastic film comprising, an extrusion diehaving an inlet, a plurality of spaced channels in said diecommunicating with said inlet, an annular outlet communicating with saidchannels, said die being adapted to receive a plastic melt in said inletwhich is separated into spaced strands in said channels and combinedprior to exiting said outlet, means providing said plastic melt to saidinlet under pressure causing the extrusion of a plastic tube closeableat a position spaced from said outlet, means for supplying a gas throughsaid die to inflate said tube to form a plastic bubble which is used todefine said film, heat transfer means in said die comprising a pluralityof spaced passages extending adjacent and parallel to said spacedchannels, and a system for providing a heat transfer fluid at acontrolled temperature and flow through said heat transfer means tocontrol the temperature of said melt by either heating or coolingthereof and thereby more precisely control the flow of said melt.
 2. Anapparatus as set forth in claim 1 in which said spaced channels comprisea plurality of parallel channels extending axially through said die. 3.An apparatus as set forth in claim 2 in which said die has an inletportion, an outlet portion, and a central portion arranged therebetweenand said plurality of spaced passages extend through said centralportion.
 4. An apparatus as set forth in claim 1 in which said systemcomprises a system for providing a liquid which is capable of eitherheating or cooling said melt.
 5. An apparatus for making plastic filmcomprising, an extrusion die having an inlet and an annular outlet,means providing a plastic melt to said inlet under pressure causing theextrusion of a plastic tube closeable at a position spaced from saidoutlet, means for supplying a gas through said die to inflate said tubeto form a thinwalled plastic bubble which is used to define said film,heat transfer means provided as an integral part of said die forcontrolling the temperature of said gas passing through said die, and asystem for providing a heat transfer fluid at a controlled temperatureand flow through said heat transfer means to control the temperature ofsaid melt by either heating or cooling thereof and thereby moreprecisely control the flow of said melt.
 6. An apparatus as set forth inclaim 5 in which said system comprises a system for providing a liquidwhich is also capable of either heating or cooling said gas.
 7. Incombination: a die for extruding plastic film having an inlet, anannular outlet, and integral heat transfer means comprising a pluralityof liquid-coveying passages in said die; means providing a plastic meltunder pressure to said inlet causing the extrusion of a plastic tubefrom said outlet; said tube being closeable at a position spaced fromsaid outlet; means for supplying a gas through said die to inflate saidtube to form a plastic bubble which is used to define said film; aplurality of gasconveying passages in said die for receiving gas exitingsaid bubble, said gas-conveying passages being arranged adjacent andparallel to portions of said liquid-conveying passages; a heat transferliquid; a system for providing said liquid through said liquid-conveyingpassages and heat transfer means; and means for controlling thetemperature and flow of said liquid to control the temperature and flowof said melt while simultaneously heating said exiting gas to atemperature which prevents cooling of said melt upon flowing saidexiting gas adjacent said melt and out of said die.
 8. A die forextruding plastic film, comprising an inlet opening for receiving aplastic melt under pressure, a passage communicating with said inletopening, a plurality of spaced channels having inlet ends communicatingwith said passage, and an annulus having an inlet end communicating withthe outlet ends of said channels and a discharge orifice for extrudingthe melt in tubular form, a first series of passages through the dieextending inwardly between said channels and thence out of a portion ofthe die surrounded by said annulus, said first series of passages beingadapted to supply heated gas continuously to inflate the tubular filmemerging from the orifice, and a second series of passages extendingthrough a portion of the die surrounded by said annulus and thenceoutwardly between said channels, said second series of passages beingadapted to withdraw gas continuously from within the inflated tubularfilm and thereby control the gaseous content of the inflated film. 9.The die of claim 8, in which each gas passage is substantiallyequidistant between the pair of adjacent channels through which itpasses.
 10. The die of claim 9, in which the gas inlet passages and gasoutlet passages pass between different pairs of channels.
 11. Anapparatus for making plastic film, comprising; an extrusion die havingan inlet; a passage in said die communicating with said inlet; astranding chamber in said die adjoining the peripheral edge of saidpassage; a plurality of spaced substantially axially extending channelsin said die communicating with said stranding chamber and each having adischarge end; a combining chamber in said die communicating with thedischarge end of each spaced channel; each of said chambers having anundulating peripheral surface defined by a plurality of substantiallyV-shaped surfaces interconnected at their bases by arcuate surfaces witheach arcuate surface being aligned with an associated end portion of anassociated chaNnel; an annular discharge orifice in said diecommunicating with said combining chamber; means providing a plasticmelt to said inlet under pressure causing the extrusion of a plastictube from said orifice which is closeable at a position spaced from saidoutlet; said plastic melt being received in said inlet as a solid columnwhich is spread radially in said passage, separated into spaced strandsin said channels, combined in said combining chamber to effectivelyminimize uneven flow rates in different parts of said die, and extrudedfrom said orifice to define said plastic tube; said V-shaped surfaces insaid stranding chamber providing smooth converging flow of plastic meltinto said channels and said V-shaped surfaces in said combining chamberproviding smooth diverging flow of said melt into an annular passagecommunicating with said annular discharge orifice; means for supplying agas through said die to inflate said tube to form a plastic bubble whichis used to define said film; heat transfer means in said die comprisinga plurality of spaced passages extending adjacent and parallel to saidspaced cahnnels; and a system for providing a heat transfer liquid at acontrolled temperature and flow through said heat transfer means tocontrol the temperature and flow of said melt.